Pulsatile release of oxytocin during the estrous cycle, pregnancy and parturition in sheep.

Oxytocin has been measured in peripheral plasma from chronically catheterized sheep both during estrus and the early luteal phase of the estrous cycle and also during pregnancy at 122-124 days gestation, 141-145 days gestation and during spontaneous labor at term. In each experiment, blood samples were collected at 5-min intervals for 120 min and also at 1-min intervals for 10 min. In pregnant and nonpregnant sheep, pulsatile patterns of oxytocin concentrations were found in the experimental periods. Mean concentrations of oxytocin were significantly higher in plasma obtained during the luteal phase of the cycle when compared to samples taken during estrus. Moreover, concentrations of oxytocin were increased with length of gestation and labor. It is concluded that oxytocin secretion is episodic in sheep during both the estrous cycle and pregnancy.